Forces in pseudopotential molecular calculations

The forces derived from the Hellmann-Feynman theory are calculated for a molecule, using the local density approximation and within the pseudopotential scheme. We show that, in contrast to the general result of all-electrons calculations, the pseudopotential scheme allows us to obtain accurate and reliable forces with a small number of basis functions. Fast convergence in the basis set is crucial for calculations of large molecules. We apply our method to the Na2 molecule and discuss the convergence rate in detail. © 1984 American Institute of Physics.